The conventional color camera system using the 525 line NTSC format performs acceptably in the studio where highly artificial and purposely designed conditions, including high intensity lighting, are provided. Since the advent of high definition color television (HDTV), the color camera has been a major problem. The major source of this problem has been in the achievement of proper registration between colors at the high resolution necessary. It is difficult enough to register three tubes in a 525 line camera, but to register 1125 television lines in an HDTV camera is a far more difficult problem.
Indeed, even in a 525 line color camera, where the registration technique has been refined over the years because of the great numbers of television cameras that have been designed, it is still necessary to use special enhancement circuitry in order to sharpen the image because exact registration is not possible. The necessity for such enhancement can be demonstrated on any 525 line camera by simply switching off the image enhancer. Immediately, the image on the television monitor becomes quite visibly soft, as a result of the fact that perfect registration is not being achieved and the enhancer is necessary to sharpen the camera video. This problem of course becomes much worse at the 1125 television lines of HDTV. Here the camera requires extremely sophisticated and complex electronic circuitry in order to achieve the level of registration demanded by 1125 television lines. The image enhancer also requires a much higher level of performance. All these factors combine to push the cost of an HDTV camera into the six figure range.
In addition, if we consider the generation of color television pictures outside the essentially laboratory (and extremely uncomfortable) conditions of the studio, as yet unsolved problems are presented in even contentional 525 line systems. This has been so in spite of the fact that television stations frequently televise programs originating away from the studio. For instance, electronic news gathering (ENG), by its nature, takes camera crews away on location, where control over lighting conditions may be nonexistent. Poor lighting may result because of an insufficient amount of natural light or the failure or inability to provide adequate supplementary lighting. Even if supplementary lighting is provided, its effectiveness is lost at ranges further than twenty feet.
An example of a situation where the problems associated with reduced lighting conditions are confronted is the televising of a military battle at night. The employment of supplementary lighting may be forbidden or impossible. The camera crew must maintain distances much greater than twenty feet as a necessary precaution under the circumstances. In addition, there are many other situations where ENG camera crews are severely restricted by reduced lighting conditions on location.
In addition, the high levels of light in the studio and the discomfort and expense associated therewith, are a manifestation of the same problems associated with conventional cameras, although such manifestation takes a very different form, namely, instead of the inability to televise in a low light level, there are the discomforts associated with high light levels.
The conventional color camera not only limits the camera crews televising opportunities, but, due to the requirement of, albeit relatively ineffective supplementary lighting, the crew's mobility is usually hampered. For instance, the crew may require one or two additional persons to set up or carry the lighting equipment. Lighting is usually hand held and is powered by rechargeable battery packs that are secured around the waist. Since the lighting time duration is limited, a number of battery packs must be available to cover an evening's assignment. It also results in the loss of coverage of an area which is too far away for the lighting to be effective, but would produce good video pictures with a low light level camera.
In an attempt to alleviate the problems attendant to televising in poorly lighted locations, low light level black and white pickup tubes were developed. These tubes have substantially greater sensitivity to light due to the provision of a image intensifier which essentially amplifies the existing received rays of light. One would think that such a solution could be rays of light. One would think that such a solution could be directly applied to a conventional color camera, except for the fact that these devices do not preserve color information. Likewise, even if devices could be used with three monochrome cameras, the above discussed registration problems would be aggravated.
More particularly, despite the longstanding availability of low light level tubes, ENG camera crews continue to use conventional color cameras and supplementary lighting. The reason for this is that together with the increased sensitivity to light, the tubes experience a high sensitivity to stray magnetic fields, even including the earth's magnetic field. Therefore, stray external magnetic fields in a three tube cluster will result in an unacceptable geometric distortion of the light image on the tube. Such geometric distortion causes unacceptable misregistration in an array of a number of low light level pickup tubes.
To understand this, consider that the conventional color camera generates three separate and independent video signals each consisting of either a red, green or blue component of the entire televised scene. It accomplishes this by employing three camera pickup tubes, one for each signal to be generated. The process could be initiated by receiving an image from the scene to be televised through a single camera lens. The image could then be triplicated by employing beamsplitters or other optical elements between the lens and the three pickup tubes. Monochromatic color filters are disposed between the beamsplitters and each of the pickup tubes for permitting only a red, blue or green images to be received by each tube. The received images are then transformed into video signals by a raster scanned electron beam in each pickup tube. The scanning format employed in the United States is a 525-line raster at thirty frames per second interlaced two to one resulting in a field rate of sixty fields per second. The scanning occurs simultaneously in each tube, and the result is three simultaneous video signals. The system described above could be referred to as a simultaneous color television system.
To achieve maximum resolution the three images on the camera tubes must be superimposed over each other exactly. This is accomplished by employing matched beam deflection and control circuitry among the pickup tubes and also matching several characteristics of the tubes. Scanning in each of the tubes is thereby made as uniform as possible and good registration and convergence is sought.
If one considers the use of three image intensified low light level pickup tubes, instead of the conventional pickup tubes, in addition to the above problems the situation is compounded by the following additional problems caused by the image intensified pickup tubes. Due to their high sensitivity characteristics, low light level tubes experience individual variations in their response. In addition, the image intensifier is highly susceptible to external magnetic fields, including the earth's magnetic field. Still further, it is noted that due to their increased sensitivity to magnetic fields and different positions in the array, resulting geometric distortion is different in each tube. Therefore, the level of color registration by arraying low light tubes together would be quite limited. Moreover, the fabrication of a low light level camera system by the employment of existing low light level technology in the classical color camera configuration has not been employed, and ENG and other industries continue to use other techniques, despite their drawbacks.
In an attempt to address these problems, about two years ago, Sony Corporation of Japan introduced a low light level color camera. The camera was tested on some shows that were televised outdoors at night using no artificial illumination but has not been accepted due to insufficient low light capability, poor resolution (under 400 television lines) and poor signal-to-noise ratio.
The Sony camera operates by using three camera sensors with an intensifier on each. However, the intensifier gain is limited to a maximum of ten, in order to achieve a reasonable degree of registration. As noted above, even with this low intensifier gain, registration was low, leaving much to be desired. In addition, the output of the intensifier converted the election image to a visible light image on a phosphor screen and then was coupled directly to the faceplate of the image sensor, resulting in poor resolution and poor signal-to-noise ratio.
Yet another approach to the provision of a full color television signal is disclosed in an article entitled Low-Light-Level Image-Amplifying Device with Full Color Capability by Stern et al published in the Journal of the Society of Motion Picture and Television Engineers, Volume 83, No. 3 which discloses a light amplifier. Here, means are provided for generating a bright full color phosphorescent image which is photographed using a motion picture film camera. While this article does mention the possibility of using a television camera instead of the motion picture camera, nothing further is said in this regard and the problem of intercolor registration is not addressed. However, the authors do note several other problematic aspects of the system, including, color distortion due to phosphor characteristics, sensitivity losses due to the use of two synchronized color filter wheels, and mechanical syncchronization of two color filter wheels with diametrically opposite filter configurations and which therefore cannot even rotate about the same axis.